05/02/2024
TREE IDENTIFICATION TIPS: BLACKGUM, NYSSA SYLVATICA
Blackgum, or black tupelo, Nyssa sylvatica, is a common tree species of the Alabama Piedmont, and it's especially common in Randolph County. However, it often goes misunderstood and unrecongized, so I thought I'd share a few identification tips here. Blackgum also serves as an instructive example for how to identify plant species in general.
Before going any further, it's necessary to dispel the mistaken assumption that blackgum is closely related, or at all very similar, to the much maligned American sweetgum, Liquidambar styraciflua. Sweetgum is usually disparaged for being of little commercial value and especially for producing spiky seed capsules that are a nuisance around home landscaping. Although blackgum is also of little commercial value, it does not produce anything resembling sweetgum's spiky seed capsules. Blackgum, unlike sweetgum, is dioecious, meaning that male and female flowers occur on separate trees. Only the female flowers produce seed, and the seeds are delivered not in dry capsules but small black-purple berries that are much less of a menace.
Here we are going to focus on the leaves, since that is how we most commonly encounter these trees. The leaves of blackgum are rather inconspicuous, unremarkable, and easily confused with other tree species, so identification often requires close examination.
Blackgum has simple leaves. The leaf blade shape varies from narrowly to broadly oval (i.e. broadly to narrowly elliptic), but also typically widest close to but just above the middle (slightly oblanceolate)--see photo A. This is a very common leaf shape and it is not immediately recognizable as blackgum. The same description can also be applied to species like American persimmon, Diospyros virginiana, or swamp blackgum, Nyssa biflora, and also many others.
A key trait to look for are the leaf tips. Blackgum leaf tips initially taper abruptly but then more gradually to form a narrow and slender point, usually forming slightly concave sides (acuminate)--see photo C. In contrast, note how the pictured swamp blackgum leaves have tips which taper more consistently to a point with more or less straight lines and without forming the concave sides (acute)--see photo B. By comparison, the blackgum leaf tips in photo C look "pinched". This is usually an effective way to distinguish between these two species at a glance, but it's also something which usually requires multiple leaves to be present.
An important character to look for when identifying blackgum leaves are the irregular marginal teeth. Usually, black gum leaves have smooth and untoothed margins with a single acuminate point at the tip. The margins are sometimes slightly wavy when viewed along a plane (undulate) and have short hairs projecting outward (ciliate), but they're not otherwise especially remarkable. However, most blackgum will have at least some leaves with irregular marginal teeth like that pictured in photo D. These teeth almost always occur exclusively above the middle of the leaf blade, they rarely number more than two or three, and they're often asymmetric. The presence of these irregularly toothed leaves is highly indicative of blackgum (i.e. Nyssa sylvatica in particular), but unfortunately they are not reliably present on every plant. They're most common on young plants and vigorous shoots but may difficult to find on older mature trees.
The last identifying trait here has to do with the arrangement of the leaves along the stem (phyllotaxy). Blackgum, like many plants, has an alternating leaf arrangement. The point along a twig that a leaf grows from is called a node. With an alternating phyllotaxy, there is only one leaf at each node, and the leaves alternate side-to-side. This can be contrasted with an opposite leaf arrangement, where there are two leaves on opposite sides of the twig at each node. When there are more than two leaves at each node, then we say that the leaves are whorled.
Most blackgum leaves alternate quite normally along the stems. However, a characteristic trait of blackgum is a pseudo-whorl of leaves at the very tips of the branches immediately beneath the terminal bud. These pseudo-whorls are especially evident with new growth, like that pictured in photo E (also check out picture A again). Technically, this is an alternating phyllotaxy, but the nodes here are so close to each other that the leaves may appear to be whorled from even a moderate distance. This pseudo-whorl is a particularly good way to distinguish blackgum from American persimmon, because American persimmon will exhibit a much more obviously alternating leaf arrangement all the way to the branch tips.
Alright, hopefully that made some kind of sense. Any questions?
